Referring to FIGS. 1 and 2, enclosed belt conveyors 1 are utilized for the safe, gentle and efficient handling of dry bulk granular materials. Totally enclosed belt conveyors have the advantage of containing dust and spilled materials and automatically reloading spilled material back onto the belt. In addition, they protect conveyed materials from contamination and degradation from environmental factors. Enclosed belt conveyors often incorporate a plurality of spool shaped idler rollers 24 to provide a trough shaped support for the conveyor belt 22. The use of spool shaped idler rollers provides a contoured support for the continuous belt which carries granular materials 31.
Enclosed belt conveyors typically include a head section 12, a tail section 14 and a number of sections intermediate between the head section and the tail section. Typically, the motor and drive mountings are located at or near the head or discharge end of the conveyor. A tail (or non-drive end) section is located at the inlet end of the conveyor. In addition, any number of intermediate sections may be placed between the head section and the tail section to create a complete conveyor assembly. An endless belt runs from the tail section to the head section, and loops around a pulley and a shaft at each end. Enclosed belts conveyors may be several hundred feet long. Consequently, the endless belts will be approximately double the length of the conveyor assembly.
All belts stretch over time. The belts used in enclosed belt conveyors tend to stretch more dramatically during their initial break-in period. However, belts continue to stretch throughout their life. One way to compensate for belt stretch, in an otherwise still usable belt, is to cut out a section of the belt and to resplice the cut ends together thus shortening the belt. Shortening a belt by splicing is a time consuming and labor-intensive process that requires that the conveyor be inoperative while the splicing is performed. It is not uncommon for a long belt to stretch several feet during its life. Therefore, it is necessary to provide for adjustment in the conveyor assembly to take up the slack that develops in a belt as it stretches and also to provide adjustment when the belt is shortened.
Although a take-up may consist of a simple movable bearing mount for the shaft and pulley it is complicated in this instance by the requirement to maintain a fully enclosed conveyor structure.
Referring to FIG. 1, a typical prior art belt tensioner used with an enclosed belt conveyor includes an inner sleeve attached to the tail end of the conveyor and an outer sleeve that is part of a tail end section of the conveyor. The inner sleeve 5 telescopes into the outer sleeve 6 and is supported by the outer sleeve. The tensioner further includes four long bolts, threaded rods or screws. Two of these long bolts are typically referred to as tail take-up screws 2 and the other two as leveling screws 4. The tail take-up screws are located on the centerline of the tail shaft bearings 7. The tail take-up screws 2 are tightened equally to tension the belt and to compensate for belt stretch. The tail take-up screws 2 must be tightened equally to prevent misalignment of the pulleys that cause belt-tracking problems. The tail section leveling screws 4 are typically located above the centerline of the tail shaft bearings. The leveling screws 4 are used to level the tail section to compensate for misalignment that occurs when the tail take-up screws 2 are adjusted to properly tension the belt and to compensate for belt stretch. The leveling screws 4 are not used for tightening the belt. The above described arrangement is typically referred to as a sliding box tail. A take-up section may also be located at the head end of the conveyor if desired or rarely at an intermediate location.
Other prior art conveyors utilize take-up sections with sliding bearing mounts and take-up screws. In this arrangement, the bearings for the end roller in the head or tail section are mounted in a plate that is slidably mounted to the conveyor housing. A screw mechanism is used to slide the plate axially related to the longitudinal axis of the conveyor to tension and compensate for slack in the belt. The length of the section in which it is located limits the adjustment distance available in a sliding bearing mount arrangement.
All of these prior art take-up sections have the disadvantage that the weight and tension of the conveyor belt and its contents are born by the housing of the take-up section. Therefore, the take-up section housing must be very robustly constructed to resist the tension of the belt, the weight of the housing, shaft, pulleys and bearings in addition to the tension of the belt which may be thousands of pounds. Heavy housing components are costly. Further, take-up sections that utilize both take-up screws and leveling screws require the proper adjustment of at least four bolts in order to properly adjust and position the take-up section. If these adjustments are improperly made, the belt will track improperly, wear excessively and have a shortened life. Thus, it would be a benefit to the enclosed belt conveyor industry if belt tensioning could be accomplished without the need for both take-up screws and leveling screws and if take-up sections could be constructed of lighter weight materials.